1. Field of the Invention
This invention relates to a developing device for developing an electrostatic latent image with toner powder, and, in particular, to a developing device for use in an electrophotographic copying machine and the like for developing an electrostatic latent image by a thin film of magnetic toner particles.
2. Description of the Prior Art
A developing device for developing an electrostatic latent image formed on an image carrying member such as a photosensitive member by magnetic toner particles is well known in the art. In particular, in electrophotographic copying machines, it is the recent tend to use magnetic toner particles, which include both magnetic and coloring materials, as a developer instead of a mixture of toner particles and carrier beads. When an electrostatic latent image is to be developed by such magnetic toner particles, it is first necessary to make a thin film of such magnetic toner particles and then such a film is brought into contact with or closer to the latent image to be developed.
FIG. 1 shows such a typical prior art developing device. As shown, the developing device includes a sleeve 1 which is driven to rotate in the direction indicated by the arrow A, a magnet roll 2 having a plurality of poles arranged along it periphery and a blade 3 which is supported in a cantilever fashion by a holder 4 at its top end. The blade 3 is made of a magnetic material and thus it is resiliently pressed against the peripheral surface of the sleeve 1 due to the magnetic attractive forces exerted by the magnet roll 2. The free end, or the bottom end in the illustrated example, of the blade 3 is pointed in the direction counter to the rotating direction of the sleeve 2 at the contact line between the sleeve 2 and the blade 3. It is true that, with such a structure, by supplying magnetic toner particles to the peripheral surface of the sleeve 1 upstream of the contact line between the sleeve 2 and the blade 3, a thin film of uninformly charged toner particles may be obtained on the peripheral surface of the sleeve 1 in the downstream of the contact line, and, thus, such a film of toner particles may be used for developing an electrostatic latent image.
However, it has been found that appreciable streaks are formed in the resulting thin film of toner particles when the device of FIG. 1 has been used for an extended period of time. One of the causes of formation of such streaks is local sticking of toner particles to the free end portion of the blade 3. As mentioned previously, since the free end portion of the blade 3 is lightly pressed against the peripheral surface of the sleeve 1, toner particles may be locally stuck to the free end portion which is in sliding contact with the sleeve 1 under pressure. Such patchy toner particles stuck to the free end of the blade 3 could cause streaks to be formed in the toner thin film. Formation of such streaks is undoubtedly disadvantageous in the developing process.